As is well known in the art, hot runner injection molding systems include a manifold for conveying pressurized melt from an inlet to one or more manifold outlets. Each manifold outlet leads to a nozzle, which, in turn, extends to a gate of an injection mold cavity. Manifolds have various configurations, depending upon the number and arrangement of the nozzles and the corresponding injection mold cavities.
It is known to heat the manifold in order to maintain a desired temperature distribution throughout the manifold. Means of heating manifolds include integrally casting or brazing an electrical heating element into the manifold, as described in U.S. Pat. No. 4,688,622 to Gellert and U.S. Pat. No. 4,648,546 to Gellert, respectively. The heating element may also be mechanically joined to the manifold by pressing the element into the manifold to create an interference, friction or deformation fit. Alternatively, thermal spraying techniques may be employed to bond the heating element to the manifold. Further, a cartridge heater may be cast in the manifold, as disclosed in U.S. Pat. No. 4,439,915 to Gellert or a plate heater may be positioned adjacent the manifold to provide heat thereto, as disclosed in U.S. Pat. No. 6,447,283 to Gellert.
Referring to FIG. 1, a typical prior art manifold is generally indicated at 100. The manifold 100 includes a manifold channel 102 and an integrated heating element 104. Heating of the manifold 100 by the heating element 104 is generally not uniform. None of the prior art manifold heating techniques provide an even heat distribution throughout the manifold. Hot spots occur at locations where the watt density is high and there is little or no contact with the surrounding mold plates. It is therefore desirable to remove heat from the manifold at these hot spot locations. As is clear from the layout of the heating element, the watt density varies from one manifold location to the next. Certain locations, near the nozzles for example, receive more heat because there is a greater length of heating element concentrated in those regions. Increasing the amount of heat generated at a particular manifold location by providing additional heating element length is generally not a practical solution. The heating element can only withstand a certain bend radius and must avoid connection points to other injection molding apparatus components such as the nozzles and the manifold backing plate. The hot/cold transition of the heating element, which is located near the entry and exit point of the heating element, is an example of a location where less heat is generated.
In an injection molding apparatus, contact between the manifold and the mold plates results in heat loss from the manifold. The location of cooling lines in the mold plates can influence the amount of heat loss from the manifold. Generally, the closer the cooling lines are to the manifold, the greater the heat loss from the manifold. Contact between the manifold and the nozzles may cause the manifold to either lose heat or gain heat depending on the particular application.
The temperature of the manifold is further influenced by the melt stream itself. For example, the temperature of the melt tends to be higher at locations where the melt experiences high shear stress, such as at bends in the manifold channel. Different types of melt will also influence the manifold temperature in different ways.
An uneven distribution of heat in the manifold causes the temperature of the melt entering the nozzles to vary slightly from one nozzle to the next. Any variation in the temperature of the melt entering each of the nozzles can adversely affect the quality of the molded products being produced by the injection molding process. With the increased use of more difficult to mold plastics materials, the melt must be maintained within narrower and narrower temperature ranges. If the temperature rises too high, degradation of the melt will result, and if the temperature drops too low, the melt will clog in the system and produce an unacceptable product. Both extremes can necessitate the injection molding apparatus being shut down and cleaned out, which can cause a very costly loss of production.
An uneven distribution of heat in the manifold has a further disadvantage in that the manifold is subjected to high stress due to continuous cycling between higher and lower temperatures. This can result in a shorter manifold life and increased downtime for the injection molding apparatus.
It is therefore an object of the present invention to provide a heat dissipation device for a manifold that obviates or mitigates at least one of the above disadvantages.